This invention relates generally to the synthesis of substituted thiophenes and, more particularly, to 3-carbomethoxy-4,5-dimethylthiophene, which is an intermediate to a family of fungicides. In another aspect, the invention relates to a novel compound, which is a precursor of the thiophene intermediates.
One fungicide that may be made from the subject intermediate is 4,5-dimethyl-N-2-propenyl-2-(trimethylsilyl)-3-thiophenecarboxamide, which is claimed as a novel compound in U.S. Pat. No. 5,486,621. Two methods of making such a compound are provided therein. Another method is described in U.S. Pat. No. 5,498,630, which concerns more generally controlling Take-All disease of plants by applying a fungicide. One related thiophene fungicide is described in Example 271, along with the method of making such compound.
A number of methods of making thiophene compounds have been disclosed. One method that is related to the method of the present invention is described, for example, in SYNLETT, Nov. 1995, p. 1143, by G. M. Coppola, R. E. Damon, and H. Yu. That method reacts an alpha-mercaptoketone with a phosphorus-substituted acrylate in the presence of a base to form a substituted dihydrothiophene ring, from which the corresponding aromatic thiophene ring can be made. It has been believed that the presence of a phosphorus-containing moiety at the 2-position was necessary for the cycloaddition reaction to occur. It has been found that it is not required, but when the 3-position is substituted, the reaction is possible in the presence of certain bases. The new method may be applied to produce thiophene rings having various substituents, including those of the titled compound, as will be seen in the description below.